a) Field of the Invention
The present invention relates to an optical head assembly used for recording to and reproducing from optical recording media.
b) Description of the Related Art
Optical head assemblies that reproduce from optical recording media such as CDs and DVDs are configured such that an objective lens converges laser light from a laser light source on a recording surface of an optical recording medium, and a photodetector detects returning light from the optical recording medium to reproduce data recorded on the optical recording media based on the detection result of the photodetector. An optical head assembly disclosed in JP Tokkai H7-302433 is configured such that a diffraction grating is placed on an optical path between the laser light source and objective lens, and thereby the laser light emitted from the laser light source is split in multiple laser beam and guided to the optical recording medium. Also, optical head assemblies disclosed in JP Tokkai H7-130021 and Tokkai H7-130223 are configured such that returning light from the optical recording medium is diffracted by a diffraction grating and guided to the photodetector.
In the optical head assembly configured in the above manner, as illustrated in FIG. 7, a recess portion 82 which is an alignment guide for mounting the diffraction grating 18 is formed in a diffraction grating 18 mounting region 81 formed in the optical head assembly to which the optical element is installed. By dropping (fitting) the diffraction grating 18 in the recess portion 82, an inner wall of the recess portion 82 and an outer peripheral surface 101 of the diffraction grating 18 fit properly to determine the mounting position of the diffraction grating 18.
Also, as illustrated in FIG. 8, the mounting position of the diffraction grating 18 may be determined by forming a diffraction grating 18 mounting region 91 on a top surface (in the figure) of the optical head assembly, to which the optical element is installed, as a flat portion, forming alignment marks 92 at the mounting region 91 on a flat portion of the optical unit, and aligning the outer peripheral surface 101 of the diffraction grating 18 with the alignment marks.
Such a diffraction grating 18 conventionally has a wide grating interval; therefore, a slight displacement of the mounting position of the diffraction grating 18 does not affect performance of the optical head assembly. Recently, however, a trend moves toward an arrangement of diffraction grating 18 and optical elements such as a photodetector in a small space by making a narrower grating interval for the diffraction grating 18 to diffract laser light at a larger angle. Consequently, performance of the optical head assembly came to be greatly dependent upon the mounting position of the diffraction grating 18. With conventional technologies, such as an alignment structure (see FIG. 7) that utilizes the outer peripheral surface 101 of the diffraction grating 18 and the inner wall of the recess portion 82 or an alignment (see FIG. 8) between the outer peripheral surface 101 of the diffraction grating 18 and the alignment marks 92, variations in outer dimension and shape of the diffraction grating 18 are immediately reflected to the displacement of the mounting position. As a result, the returning light from the optical recording medium may not be efficiently guided to the photodetector; thus, performance of the optical head assembly may be degraded.
The present inventor has filed applications of new types of optical pickup apparatus (Tokugan H8-167037, Tokugan H9-125128) in which, by using a diffraction grating having a diffraction pattern of irregular interval (modulating diffraction grating) to obtain highly precise error signals from the returning light, light emitted from the laser light source is split into three beams and guided to a converging lens, and focusing error signals are detected from the returning light from the recording surface of the recording medium. In these optical pickup apparatus, a focal length of a beam on the recording surface of the recording medium is changed by adjusting the grating interval of the diffraction grating. Also, the distance between adjacent beams of the three beams is determined by a grating position with respect to the laser light source. However, the modulating diffraction grating has a diffraction pattern of irregular interval. If the positional relationship between the grating position of the diffraction grating and the laser light source is broken in the optical head assembly which uses the above method, a predetermined focal length and beam distance cannot be obtained. Therefore, in the optical pickup apparatus having the modulating diffraction grating, a precise alignment of the diffraction grating is required. However, the conventional alignment structures cannot provide a precise in alignment to meet such a requirement.